Stereoscopic images generally consist of two images which are related by a small change in the lateral perspective. When viewed through an enabling apparatus, stereoscopic images may provide the perception of stereoscopic depth. Anaglyphs are stereoscopic images wherein different sets of primary colors are used to render the first and second images of the stereo pair. Usually, the spectra of the first and second images do not overlap significantly. Then the first and second images may be viewed selectively using two complementary color viewing filters. The first viewing filter F1 may be used to view the first image while the second viewing filter F2 may be used to view the second image. The first filter substantially transmits the primary colors of the first image and blocks the primary colors of the second image. The second filter substantially transmits the primary colors of the second image and blocks the primary colors of the first image.
Anaglyphs are often rendered in three primary colors where the first image is rendered in two primary colors while the second image is rendered in one primary color. In red/cyan anaglyphs, the first image is rendered in green and blue primary colors while the second image is rendered in a red primary color. Other types of anaglyphs include blue/yellow and green/magenta anaglyphs. Herein these anaglyphs are called three-color anaglyphs.
Anaglyphs are often used to display stereoscopic images due to their relatively low cost and wide compatibility with display devices. However, conventional anaglyphs have some well known disadvantages. Firstly, conventional anaglyphs generally exhibit a reduced color gamut when viewed through the colored viewing filters. Secondly, conventional anaglyphs generally exhibit retinal rivalry which may cause user discomfort. The prior art contains many methods to improve the color gamut of anaglyphs. The prior art also contains many methods to reduce the retinal rivalry in anaglyphs. However, these anaglyphs still have reduced color gamuts and exhibit retinal rivalry.
It is commonly known that viewing a subject through colored filters may reduce the observed color gamut of the subject. In general, a color filter which transmits only a single primary color may not allow any color hue to be fully perceived through the filter. For example, an image rendered in a pure red primary color may appear to be nearly a grayscale image when viewed through the red filter.
On the other hand, a filter which transmits two primary colors may allow only the hues associated with the two primary colors to be perceived through the filter. The hue consisting of both primary colors may appear to be nearly a gray color through the filter. For example, a cyan filter (which transmits green and blue light) may allow only blue and green hues or blue and greenish-yellow hues to be perceived through the filter depending on how close the green primary color is to yellow. An image rendered in pure cyan hues may appear to be nearly a grayscale image when viewed through a cyan filter. These phenomena may be confirmed by viewing a digital color spectrum through pure cyan and pure red filters. Software programs for editing digital images often provide a suitable digital color spectrum in their color selection tools.
Since the second image in an anaglyph, is generally perceived as a grayscale image, the color gamut observed in a stereo view of an anaglyph is generally similar to the color gamut of the first image rendered in two primary colors. The first image in an anaglyph generally contributes more to color perception than the second image.
From these observations, one might expect that only blue and yellowish-green hues may be perceived in red/cyan anaglyphs. However, additional color hues are often visible in conventional red/cyan anaglyphs.
One common method of creating red/cyan anaglyphs is to combine the green and blue primary channels of the first image with the red primary channel of the second image. This type of anaglyph is often called a “true-color” anaglyph. Surprisingly, red and cyan hues may be perceived in some true-color anaglyphs when viewed through red and cyan viewing filters. In other words, while the single filters do not allow red or cyan hues to be perceived, the stereo view through the two filters may allow red and cyan hues to be perceived. However, the red and cyan hues are generally accompanied by large amounts of retinal rivalry. Similar phenomena occur in analogous blue/yellow and green/magenta true-color anaglyphs.
True-color anaglyphs generally to have too much retinal rivalry for comfortable viewing. Therefore many methods have been developed in the prior art to produce anaglyphs with less retinal rivalry than observed in true-color anaglyphs. In order to observe less retinal rivalry, anaglyphs are often constructed from images with modified colors. These color modifications may reduce the retinal rivalry observed in the anaglyph, but may also reduce the spectrum or saturation of hues perceived in the anaglyph. Herein these anaglyphs with modified colors and rendered in three primary colors are called partial-color anaglyphs.
There are various editing operations which may be applied to stereoscopic images prior to constructing an anaglyph which are known to reduce retinal rivalry. These include de-saturation of hues and hue substitution. Many methods involve local editing of an image so that the editing functions vary throughout an image. These are very labor intensive and expensive methods to prepare anaglyphs. A particular method of the prior art may cause an average reduction of retinal rivalry in a stereo view while patches of high retinal rivalry remain scattered throughout the stereo view. However, the prior art does not provide a method to reduce the retinal rivalry to arbitrarily low levels for any distribution of initial color content in a stereoscopic image. The prior art lacks a working theory of how to avoid retinal rivalry when producing partial-color anaglyphs.
The conditions which are required to avoid retinal rivalry in color anaglyphs are not described in the prior art. Generally, the prior art contemplates a compromise between the color gamut and the level of retinal rivalry observed in an anaglyph. It is widely believed that retinal rivalry is necessarily present to some degree in color anaglyphs. In order words, it is widely believed that all color anaglyphs have more retinal rivalry than grayscale images. Most efforts of the prior art have been directed toward improving the color gamut of partial-color anaglyphs while accepting a reduced but substantial amount of retinal rivalry.
Methods exist in the prior art to increase the color gamut of anaglyphs by using leaky viewing filters. It is widely known that the range of perceived hues in partial-color anaglyphs may be expanded to some degree by allowing one or both of the viewing filters to partially transmit or leak a small amount of additional primary colors through the filters. For example, a red filter which also transmits a small amount of green light may allow a dark green hue and an unsaturated red hue to be perceived through the red filter. Or a cyan filter which also transmits a small amount of red light may allow a dark red hue and an unsaturated cyan hue to be perceived through the filter.
Transmitting part of the primary colors of the opposite image through the viewing filters may cause the user to see ghost images or double images in the stereo view. The double images may reduce the ability of the user to fuse the stereo pair and may reduce the perceived stereoscopic depth in the stereo view. Therefore, when using leaky filters, the benefit of the extra hues created by the leak must be balanced against the disadvantage of perceiving less stereoscopic depth.
Conventional cyan filters for viewing red/cyan anaglyphs are often designed to leak a small amount of a red primary color through the filter. This allows a weak reddish hue to be perceived through the cyan filter. However the leaked red primary color creates a ghost of the second image in the view of the first image. Furthermore since the second image is often offset from the first image due to stereoscopic parallax, the red light from the second image is not always at the proper location to contribute correctly to the color of the first image. Similar disadvantages occur when using leaky filters with blue/yellow and green/magenta anaglyphs.
The prior art contains methods to predict the color gamut observed in anaglyphs viewed through leaky filters using conventional color models such as the CIE (International Commission on Illumination) RGB color models. The CIE color models were developed for red, green and blue primary colors. However, it is clear that color perception is drastically changed by color viewing filters. For example, a red viewing filter may change a red color, which is considered a dark color in conventional color models, into a white color which is a bright, unsaturated color. Therefore, applying conventional color model calculations to predict the color gamut perceivable through color filters has questionable meaning. Furthermore, the color gamut perceivable in an anaglyph depends on the amount and distribution of retinal rivalry. In fact, the effects of retinal rivalry on the perceived color gamut is often greater than the effect of leaking complementary colors through the filters. This is a further reason that color gamut calculations based on conventional color models have limited meaning when applied to conventional anaglyphs.
Grayscale anaglyphs are anaglyphs which are constructed from grayscale versions of stereoscopic images. The grayscale values of the first image are displayed in two primary colors while the grayscale values of the second image are displayed in the remaining primary color. A grayscale anaglyph may appear grayscale when viewed through the anaglyph viewing filters. Grayscale anaglyphs have the advantage of having nearly no perceivable retinal rivalry, but have the disadvantage of not providing colored stereo views. Herein, an anaglyph is considered to be a color anaglyph unless otherwise stated.